TM 55-1520-240-23-10
15-1
AUXILIARY POWER UNIT
(Continued)
15-1
At about 5 percent of rated speed, the ESU applies
power to the ignition exciter and opens the start fuel
valve. It also activates a 40 second timer. Fuel flows
through the start fuel valve and a start fuel nozzle into
the combustor, where it is vaporized and ignited by the
exciter-energized igniter plug. The burning fuel mixture
increases APU speed.
NO BREAK - WORK HARDER
At about 14 percent of rated speed, the ESU opens the
main fuel valve, allowing fuel to flow through the six
main vaporizer tubes into six fuel injectors. The injectors
direct fuel into the combustor, increasing APU speed.
NO BREAK - WORK HARDER
At about 20 percent, the charge in the start accumulator
is depleted and the motor-pump no longer drives the
APU.
NO BREAK - WORK HARDER
At about 70 percent, combustion is self-sustaining.
Power is removed from the ignition exciter and start fuel
solenoid.
At about 90 percent the motor-pump becomes a pump
and is driven by the APU to recharge the accumulator.
After 1.5 seconds the APU ON light on the master
caution panel comes on. If 90 percent rpm is not
attained within 40 seconds of when start power was
applied to the ESU (5 percent) rpm, a fault relay cuts
off fuel to the APU.
NO BREAK - WORK HARDER
If the temperature or speed of an operating APU
becomes abnormal, the ESU will automatically shut
it down. Overtemperature during the start sequence
is anything over a sustained 1,250ºF (677ºC). During
operation, it is anything over 1,150ºF (621ºC).
Overspeed is anything more than about 110 percent
of rated speed. The APU will also shut down if speed
drops below 90 percent 1.5 seconds after attaining
that speed.
END OF TASK
15-3/(15-4 blank)
